An internal combustion engine conventionally comprises a crankshaft rotating about a horizontal axis, pistons which slide in cylinders with reciprocating movements, and connecting rods the top ends of which are connected to the pistons and the bottom ends of which are connected to the crankshaft via eccentric connections. The reciprocating movement of the pistons, initiated by the combustion of a mixture of air and of fuel in the cylinders, thus provides the drive that turns the crankshaft.
Because of this device and because of the combustion of the mixture of air and fuel in the cylinders, such an engine is subject to vibrations which are troublesome not only to the occupants of the motor vehicle but also in terms of engine reliability.
At low speeds, when the driver of the motor vehicle demands high torque, the greatest vibrations are caused by the combustion of the fuel in the cylinders. The detonation of the air-fuel mixture in the cylinders in fact generates a “combustion vibration moment” that is very high.
At high speeds, the greatest vibrations are associated with the reciprocating movements of the piston, of the connecting rods and of the crankshaft. These movements in fact generate an “inertial vibration force” and an “internal vibration moment” which are very high.
To damp out some of these vibrations it is therefore known practice to equip the internal combustion engine with a system of balance shafts which, by rotating, generates inertial forces and inertial moments which oppose the aforementioned vibration moments and vibration forces.
An internal combustion engine comprising two identical balance shafts, arranged side by side, parallel to the crankshaft in a plane orthogonal to the axes of the cylinders is known, particularly from document FR 2 252 510 (or its US equivalent U.S. Pat. No. 3,995,610). By rotating at the same speed but in opposite directions, these balance shafts are able to counteract the inertial vibration moments and inertial vibration forces at high speeds, but are unable to counter the combustion vibration moment, which means that vibrations remain at low speeds.
Also known from that document FR 2 252 510 is an internal combustion engine comprising two identical balance shafts arranged side by side, parallel to the crank shaft, in a plane that is inclined with respect to the axis of the cylinders. Because they are arranged in the aforementioned inclined plane, these balance shafts are able to generate an additional moment which, when the inclination of the inclined plane is sufficient, counters the combustion vibration moment when the engine is operating at low speed and high load.
The disadvantage with such a system is that at high speed and/or at light load, the additional moment generated by these balance shafts is very much greater than the combustion vibration moment, which means that it generates new vibrations.
The aforementioned two balance shaft systems are therefore unable to effectively damp the vibrations for all engine operation configurations.
These balance shafts, which are situated in a plane that is orthogonal to or inclined with respect to the axes of the cylinders also require a large amount of clearance volume in which to turn, and this is incompatible with the objectives of increasing the engine compactness.